1. Field of the Invention
The present invention relates to a light unit using a point light source, and a liquid crystal display using the light unit as a front light.
2. Description of the Related Art
Reflection type liquid-crystal displays are notably mounted in portable electronics such as cellular telephones, personal digital assistants (PDA), and notebook personal computers because they allow a user to clearly view images in a light place than transmitted type liquid-crystal displays. The reflection type liquid-crystal display has a surface light source device as a light unit to provide against insufficient ambient light if it is used outdoors at night.
When an attempt is made not to see moirxc3xa9 stripes by setting a desired positional relationship between the arrangement of recesses and projections forming a prism surface of a light guide panel for use in a light unit and the direction in which pixel electrodes reflecting light are arranged, a problem occurs if a point light source is used to provide light incident on the light guide panel. The causes of this problem will be sequentially described below.
As shown in FIG. 18, a light unit 102 in a reflection type liquid-crystal display 101 is arranged on the front surface side of the display which is opposite an observer relative to a reflection type liquid-crystal panel 22 based on, for example, a TFT (Thin Film Transistor) method, in order to irradiate the liquid-crystal panel 22 with illumination light, while transmitting reflected light from the liquid-crystal panel 22, through the liquid-crystal panel so that the observer can view the light. This is called a xe2x80x9cfront lightxe2x80x9d. Line light sources using fluorescent lamps or the like have been used for the front light.
As shown in FIGS. 18 and 20, the front light 102 has a fluorescent lamp 104 shaped like a thin tube, a reflector 105 that covers three peripheral sides of the fluorescent lamp 104 in the longitudinal direction thereof to reflect light from the fluorescent lamp 104 to emit band-shaped light through an open side of the reflector, and a light guide panel 7 comprising a light transmitting material and shaped like, for example, a plate and which receives direct light from the fluorescent lamp 104 or reflected light from the reflector 105 on a facet 4 and converts the received incident light into planar light to reflect this light to the liquid-crystal panel 22, while transmitting and guiding the reflected light from the liquid-crystal panel 22 to the observer.
The light guide panel 7 has a prism surface 6 on one side thereof which has a stripe-like pattern of recesses and projections forming a plurality of linear parallel ridge lines, and a flat light applied surface 5 on the back surface thereof. Further, as shown in FIG. 19, which is an enlarged view of a portion E of FIG. 18, which is a side view of the liquid-crystal display 101, the prism surface 6 has, for example, steep slopes 12 and gentle slopes 13 which are alternatively formed. The light guide panel 7 receives band-shaped light from the fluorescent lamp 104 through a facet 4 side, converts the incident light into planar light at the prism surface 6, and then reflect it to the liquid-crystal panel 22. The light impinging on the liquid-crystal panel 22 is reflected by the liquid-crystal panel 22, transmitted through the light guide panel 7 again, and then emitted.
On the other hand, the liquid-crystal panel 22 has pixels driven by, for example, the TFT method. As shown in FIGS. 21 and 22, the liquid-crystal panel 22 has a TFT substrate 24 having a large number of TFTs 111 and pixel electrodes 112 formed in a matrix, an opposite substrate 25 fixed opposite the TFT substrate 24 via a clearance of several-pm size and having a colored layer (color filter) 115 formed thereon, a liquid crystal layer 26 sealed in the clearance, and a polarizing plate 27 and a quarter wavelength plate 28 both disposed outside the opposite substrate 25. The pixel electrode 112 is also used as a reflection member.
Moirxc3xa9 stripes, interference stripes resulting from overlapping of groups of parallel lines arranged at a predetermined pitch, may be viewed depending on how the ridge lines on the prism surface 6 overlap the pixel arrangement of the liquid-crystal panel 22, thereby degrading display quality. The reason why these moirxc3xa9 stripes are viewed will be described below. The stripe-shaped recesses and projections of the prism serve to concentrate light in one direction, thereby creating small differences in brightness along the ridge lines of the recesses and projections. On the other hand, if the pixel electrodes of the liquid-crystal display are used to reflect light, some of the clearances between adjacent pixels fail to reflect light. Since the liquid panel has the pixels arranged in a matrix and composed of the TFTs 111 and the pixel electrodes 112, as shown in FIG. 21, those areas which do not reflect light appear like stripes. If the ridge lines of the prism cross the arrangement direction of the pixels at a small angle, the above described brighter and darker areas are likely to interfere with each other when crossing each other, thereby causing moirxc3xa9 stripes as interference stripes to be viewed. These moirxc3xa9 stripes are most noticeable when the ridge lines 110 of the prism surface 6 are slightly offset from the pixel arrangement direction in the horizontal or vertical direction of the display.
Moirxc3xa9 stripes are not seen by paralleling the ridge lines of the prism and the pixel arrangement direction or setting these directions to cross each other at a large angle. However, the prism is provided in the light guide panel, the pixels are provided in the liquid-crystal panel, and the light guide panel and the liquid-crystal panel are separate components constituting the liquid-crystal display. Accordingly, the ridge lines of the prism and the pixel arrangement direction are likely to cross each other at a small angle in spite of an attempt to parallelize these directions. Thus, instead of parallelization, the ridge lines and the pixel arrangement direction may be set to cross each other at a large value.
Thus, it has been contemplated that the ridge lines may be formed at about 23xc2x0 from the direction in which the pixels are arranged in a matrix. Such a technique is described in, for example, xe2x80x9cTechnological Trend of Front Lightsxe2x80x9d by Akira TANAKA (Monthly Display, June 1999, p. 48 to 53) and Japanese Patent Laid-Open No. 2000-155315. The direction in which the pixels are arranged in a matrix is generally parallel with the longitudinal direction of the fluorescent lamp. Accordingly, as shown in FIG. 20, the ridge lines 110 are formed at an angle xcex8 of about 23xc2x0 from the longitudinal direction of the fluorescent lamp 104 and at a predetermined pitch p0.
However, if the fluorescent lamp is used as a light source, it has high power consumption and requires an inverter that generates a high voltage for lighting. Accordingly, the fluorescent lamp is an obstacle to the reduction of the size and weight of the display. To meet the demand for the reduction of the power consumption, size, and weight the display, a technique has been proposed which converts light from a point light source composed of, for example, white LEDS (light emitting diodes) into false linear light using a light guide, as described in Japanese Patent Laid-Open No. 2000-11723 or the like. Japanese Patent Laid-Open No. 10-260405, which relates to a back light, also describes the technique of converting light from LEDs as a point light source into false linear light.
A front light using such a point light source will be described with reference to, for example, Japanese Patent Laid-Open No. 2000-1723, mentioned above. As shown in FIGS. 23 and 24, a front light 201 has a point light source 2a composed of, for example, white LEDs, a light guide 3 that converts light emitted from the point light source into band-shaped light, and a light guide panel 204 comprising a light transmitting material and shaped like, for example, a plate and which receives the band-shaped light from the light guide 3 and converts the incident light into planar light to reflect this light to a liquid-crystal panel, while transmitting and guiding reflected light from the liquid-crystal panel to the observer. When the front light 201 and the liquid crystal panel are assembled into a reflection type liquid-crystal panel, for example, the longitudinal direction of the light guide 3 of the front light 201 substantially matches the pixel arrangement direction of the liquid-crystal display, and the front light 201 is disposed on the display surface side of the liquid crystal panel.
The light guide panel 204 has a large number of grooves 206 formed in a surface 205 of the light guide panel 204 along the longitudinal direction of the light guide 3, that is, the pixel arrangement direction of the liquid-crystal panel and which grooves are parallel with a stripe-like pattern. The light guide panel 204 receives band-shaped light from the light guide 3 via a facet 207 thereof, and reflects the received incident light from the surface 205 to a back surface 208 to irradiate the liquid-crystal panel with the illumination light, while transmitting reflected light from the liquid-crystal panel, through the light guide panel. Further, to increase the quantity of light, a front light 201A may be used which has point light sources 2a and 2b at the opposite ends of the light guide 3, as shown in FIG. 25. Thus, the front light 201 having the point light source 2a arranged at only one end of the light guide 3 and the front light 201A having the point light sources 2a and 2b arranged at the opposite ends of the light guide 3 have a reduced power consumption, size, and weight. Consequently, these front lights are expected to be effectively mounted in, for example, cellular telephones.
However, in the front lights 201 and 201A described in Japanese Patent Laid-Open No. 2000-11723 and using the point light source, the grooves 206 formed in the surface of the light guide panel 204 are formed generally parallel with the longitudinal direction of the light guide 3 (that is, the pixel arrangement direction). Accordingly, these front lights cannot prevent moirxc3xa9 stripes as described above, thereby possibly degrading the display quality.
Thus, an inventor contemplated that the light guide 7 having the ridge lines 110 of the prism inclined at an angle xcex8 from the linear light source 104 shown in FIG. 20 may replace the light guide panel 204 in the front light 201 (FIG. 23) or 201A (FIG. 25). The inventor experimentally produced a front light 301 using point light sources 2a and 2b and in which the ridge lines 110 of the prism surface 6 of the light guide panel 7 are inclined at the predetermined angle xcex8 from the longitudinal direction of the light guide 3, as shown in FIGS. 26 to 28.
In the front light 301, the ridge lines 110 of the prism surface 6 are formed at the angle xcex8, for example, about 23xc2x0 from the longitudinal direction of the light guide 3 as shown in FIG. 26, and at a predetermined pitch p0. Further, the light guide 3 is provided with a reflecting plate that surrounds three sides of the light guide 3. In the other points, the front light 301 is the same as the conventional technique shown in FIG. 25. Further, the prism surface 6 has, for example, the steep slopes 12 and the gentle slopes 13 which are alternately formed, as shown in FIG. 29.
As shown in FIG. 26, an incident light from the point light source 2a, arranged on the side of the light guide 3 on which the longitudinal direction of the light guide 3 and the ridge lines make an obtuse angle, is converted into band-shaped light by the reflecting plate (not shown), surrounding all the sides of the light guide other than its light entering and emitting surface. The band-shaped light is then emitted to the facet 4 of the light guide panel 7. However, this band-shaped light is polarized toward an end of the light guide which is opposite the other end thereof with the light source 2a rather than traveling through the light guide panel in a direction perpendicular to the longitudinal direction of the light guide 3, as shown in FIG. 28. This polarization occurs because when light incident on the light guide from the point light source is reflected from the reflecting plate, it is mainly reflected away from the point light source (a main irradiation direction in which light is most intense) and then impinges on the light guide panel. Thus, it is likely that a triangular area (312 in FIG. 26) is formed in which light is unlikely to reach locations close to those of the side surface of the light guide panel perpendicular to the longitudinal direction of the light guide which are closer to the point light source 2a. 
On the other hand, the ridge lines 110 are inclined through about 23xc2x0 from the light guide 3 as described above, so that the main irradiation direction V is likely to be orthogonal to the ridge lines 110. Thus, as shown in FIG. 29, when light incident in the main irradiation direction V, which is generally orthogonal to the ridge lines, impinges on the steep slope 12, which forms a recess and a projection of the prism, it is totally reflected in a generally perpendicularly downward direction W. This reflected light is transmitted through the light applied surface 5 and then totally reflected from the liquid-crystal panel 22 and generally perpendicularly to the liquid-crystal panel 22, as shown in FIG. 27. Then, the light is transmitted through the light guide panel 7 again, emitted in a generally perpendicular direction from the front surface of the front light 30, and then viewed by the observer. That is, intense light traveling in the main irradiation direction is emitted in the observer""s viewing direction and viewed by the observer as it is. Accordingly, the light travelling in the main irradiation direction is viewed as more intense light than light travelling in the other directions. Consequently, only the light travelling in the main irradiation direction, which is generally orthogonal to the ridge lines, becomes intense and is viewed by the observer after reflection. Owing to a combination of the facts that certain areas appear as intense light and that a triangular area is formed in which light is unlikely to reach certain locations, stripe-shaped lines forming brighter and darker areas are viewed on the surface of the light guide panel, thereby degrading the display quality.
Furthermore, the main irradiation direction VO of light incident from a corner 310 of the light guide panel 7 which is closer to the point light source 2a is slightly polarized from the perpendicular direction y of the display toward the end of the light guide 3 which is opposite the other end with the point light source 2a. Further, virtually no light is incident from a facet 311 of the light guide plate 6 which is perpendicular to the facet 4 thereof and which is located closer to the point light source 2a. Consequently, as shown in FIG. 26, a generally triangular relatively dark area 312 is formed, thereby disadvantageously making bright lines conspicuous.
If a fluorescent lamp is used as a light source such as the one shown in FIG. 20, the phenomenon in which brighter and darker areas composed of stripe-shaped lines appear is also observed, but to an acceptable degree. In contrast, the phenomenon in the case of the front light 301 using the point light source shown in FIG. 26 affects the display quality. In this case, the phenomenon is observed whether or not another point light source 2b is present. That is, even when the inventor tested a front light 301A using a single point light source as shown in FIG. 30 in order to further reduce the power consumption, size, and costs of the display, brighter and darker areas composed of stripe-shaped lines appeared, as in the case with the front light 301 having the point light sources arranged on the opposite sides of the light guide.
As shown in FIG. 31, a diffusion member 31 of a predetermined thickness may be interposed between the light guide 3 and the light panel 7 to level off the peak of the intensity of light travelling in the main irradiation direction, in which the light is most intense. However, the peak in the main irradiation direction V can be restrained only to the extent that light of a required intensity travels throughout the light guide panel 7, resulting in insufficient diffusion. Consequently, stripe-shaped lines are still viewed. Thus, it has hitherto been impossible to reliably restrain the degradation of display quality associated with non-uniform luminance.
The present invention is provided in view of these points, and it is an object thereof to provide a front light, a liquid-crystal display, and electronics which can reliably reduce the occurrence of non-uniform luminance to improve display quality.
It is thus an object of the present invention to provide a further improved light unit.
It is another object of the present invention to provide a light unit having an improved structure comprising a point light source and a plate-shaped light guide having a prism disposed thereon and formed of recesses and projections having ridge lines.
It is further another object of the present invention to provide a light unit having a structure that can eliminate the non-uniformity of luminance even if a point light source is used, when false linear light into which light from the point light source is converted by a columnar light guide.
It is further another object of the present invention to provide a light unit having a structure that can eliminate the non-uniformity of luminance when false linear light into which light from the point light source is converted by the columnar light guide even if the angle between the direction in which pixels are arranged on the liquid-crystal display in a matrix and the ridge lines of the recesses and projections forming the prism formed on the surface of the plate-shaped light guide is set to have a predetermined value.
A light unit according to the present invention comprises a columnar light guide that reflects light emitted from a point light source, to emit generally band-shaped light, and a rectangular plate-shaped light guide that receives the light emitted from the columnar light guide, through a facet thereof, and reflects and refracts the light received through the facet, using a prism surface thereof having recesses and projections formed thereon and forming a plurality of linear parallel ridge lines, so that planar light is emitted from a light applied surface thereof which is opposite the prism surface, wherein the point light source is arranged only at an end of the face of the columnar light guide at which the angle between the facet and the ridge lines is obtuse.
Furthermore, a light unit according to the present invention has a columnar light guide that receives light emitted from a point light source, through an end thereof, and reflects and refracts the received light while guiding it in a longitudinal direction thereof, to emit generally band-shaped light through a side surface thereof, and a plate-shaped light guide that receives light emitted from the columnar light guide through a facet thereof, and reflexes and refracts the light received through the facet using a prism surface having recesses and projections formed thereon and forming a plurality of linear parallel ridge lines inclined through a predetermined angle from the longitudinal direction of the columnar light guide, so that planar light is emitted from a light applied surface of the plate-shaped light guide which is opposite the prism surface, wherein the point light source is arranged only at an end of the facet of the plate-shaped light guide at which the angle between the facet and the ridge lines is obtuse.
According to this configuration, the positional relationship between the point light source and the ridge lines is set so that the main irradiation direction of band-shaped light emitted from the columnar light guide changing false linear light from the point light source is not generally orthogonal to the ridge lines of the recesses and projections forming the prism surface of the plate-shaped light guide. Consequently, light obtained by totally reflecting this band-shaped light is unlikely to be viewed, there by eliminating the non-uniformity of luminance.